Drug dependence is characterized by recurrent, uncontrollable bouts of drug use, even after prolonged periods of abstinence. This suggests enduring changes occur in the brain that can last even after drug use has discontinued. A mechanism underlying long-lasting changes is altered gene expression. Recent research has focused on epigenetic alterations at the DNA level, however, little is known about the impact of post- transcriptional processes on gene expression. MicroRNAs (miRNAs), a class of small, highly conserved non- coding transcripts, control the duration and expression of target mRNAs and have been implicated in development and disease states. This exciting research area forges new ground into uncovering potential mechanisms of drug addiction. Our laboratory, in collaboration with Dr. Nora Perrone-Bizzozero, has preliminary data demonstrating that miR-495, a striatal-expressed miRNA that has several addiction-related genes as predicted targets, is downregulated in the nucleus accumbens (NAc) in response to acute cocaine, while two of its predicted targets, BDNF and arc, are upregulated. Furthermore, viral overexpression of miR- 495 in the NAc decreases BDNF and arc levels, and more importantly, reduces responding in rats during a progressive ratio (PR) schedule of cocaine reinforcement. We hypothesize that increasing levels of miR-495 negatively regulates addiction-related genes involved in cocaine reinforcement and motivation. To test our hypothesis, we will first examine the effects of virally overexpressing or knocking down levels of miR-495 in the NAc on cocaine self-administration on low ratio and PR schedules of reinforcement, and measure the associated changes in miR-495, BDNF, and arc levels in mesocorticolimbic structures. We expect to replicate our previous finding of attenuated PR responding and decreased levels of BDNF and arc after miR-495 overexpression, and we predict the opposite effect when miR-495 is downregulated. Furthermore, we plan to use the same manipulations on sucrose reinforcement to test whether our effects are specific to cocaine. Second, we will examine the effects of manipulating miR-495 levels on cocaine-seeking behavior following short and long periods of forced abstinence. Because prolonged abstinence is associated with increased motivation for drug (i.e. incubation effect), these experiments will more specifically address the role of miR-495 in drug motivation. We predict overexpression of miR-495 will decrease cocaine seeking at both time points and may prevent the incubation effect and associated changes in BDNF and arc expression, while leaving sucrose seeking unaltered. The results may suggest a novel approach to post-transcriptionally regulate several addiction-related genes thereby providing a new avenue for developing treatment for drug dependence.